Fix BadContinuationPointInvalid that occurred due to incorrect Browser.BrowseAsync

[KarenKrill]

Fix BadContinuationPointInvalid that occurred due to incorrect handling of an empty continuation point byte array in Browser.BrowseAsync

Description

Temporary hotfix of BadContinuationPointInvalid when calling Browser.BrowseAsync with servers 1.6.0 and higher.

Related Issues

• Fixes Browser.BrowseAsync treats empty ContinuationPoint (byte[0]) as valid and calls BrowseNext when ContinueUntilDone=true #3698

Checklist

• I have signed the CLA and read the CONTRIBUTING doc.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added all necessary documentation.
• I have verified that my changes do not introduce (new) build or analyzer warnings.
• I ran all tests locally using the UA.slnx solution against at least .net framework and .net 10, and all passed.
• I fixed all failing and flaky tests in the CI pipelines and all CodeQL warnings.
• I have addressed all PR feedback received.

Further comments

The issue seems to be fixed by a refactoring in the master branch (adding the ByteString type), but I'm proposing a temporary spot hotfix for latest 1.5.378 version

[marcschier]

The merge commit for this PR was removed from release/1.5.378 (the branch tip is back at 083dbe2, Prepare 1.5378 maintenance release (#3982)).

Maintenance fixes for the 1.5.378 lineage have to land on master378 first and are then picked into release/1.5.378, so this change was re-submitted against master378 in #4168, together with unit tests that pin the behaviour for null, empty and non empty continuation points.

Thanks @KarenKrill for the fix - the commit in #4168 credits you as co-author.